What are the regulations/protocols for measuring square footage of a rent stabilized (not rent-controlled) apartment in NYC?

As noted, the size of an apartment is not a factor in its status as a rent-stabilized apartment. Thus, there are no protocols or regulations with regard to the measuring of an apartment in order for an apartment to enter the rent-stabilization program. The initial rent for new apartments entering the rent stabilization program are generally determined as a factor of "affordability," as further determined by local economic conditions, average income and other factors. For example, the owner of a new rental building with 100 apartments will typically have agreed to make 20%, or 20 units, "affordable" in exchange for tax, zoning, or other benefits. Those apartments are then registered with NYC's rent-stabilization program with initial rents that have been deemed "affordable" in accordance with the necessary criteria. Market rents are generally determined not by the size of the apartment but by the number of bedrooms.  Stabilized rents are generally determined by the income (or "affordability") of the initial tenant, with the rent to be regulated thereafter for any subsequent tenant. Once the "legal rent" reaches $2,500 per month, and the apartment becomes vacant, the apartment is then in "luxury decontrol" and can exit the rent-stabilization program. Renovations of apartments can increase the "legal rent" and thereby accelerate the process towards luxury decontrol.